There is no question that in recent years environmental restrictions on permissible power plant pollutant output levels have become more restrictive. As a consequence, both the private and public sectors have been devising means and methods to reduce debilitating pollution levels.
One particularly invidious form of pollution is caused by NO.sub.x emmissions. NO.sub.x (the generic name for the oxides of nitrogen) is formed by both fuel-bound nitrogen and the free nitrogen contained in the combustion air introduced into a combustion chamber according to the following formulae: EQU 1/2N.sub.2 +O.sub.2 .fwdarw.NO.sub.2 (nitrogen dioxide) EQU N.sub.2 +O.sub.2 .fwdarw.2NO (nitric oxide)
It has been long known that the oxides of nitrogen are one of the main constituents of photochemical smog. Furthermore, nitrogen dioxide (NO.sub.2) is a dangerous substance in of itself. To put the NO.sub.x emissions problem in perspective, it has been estimated that United States power plants generate approximately 4.5 million tons of NO.sub.x a year. In comparison, mobile power sources (primarily internal combustion engines) produce an additional 8 million tons of NO.sub.x a year. Obviously, it is necessary to reduce the generation of NO.sub.x.
The production of NO.sub.x formed by various combustion schemes may be decreased by a considerable degree by modifying the underlying combustion process itself. For example, delayed combustion, staged combustion, and the vitiation of combustion air have been successfully employed in the past. Unfortunately, the NO.sub.x reduction that can be achieved by such means may not be sufficient to meet the allowable emission criteria in many localities. As a consequence, post-combustion removal of the undesirable NO.sub.x is required.
Further to the above, there are a variety of wet and dry processes that have been developed over the years for post-combustion NO.sub.x removal. The instant invention relates to a dry process known as "selective catalytic reduction". This process involves, firstly, the addition of ammonia to an exiting flue gas stream and secondly, the subsequent passage of the ammonia/gas mixture in registry with a suitable catalyst to selectively reduce the NO.sub.x to nitrogen and water.
The chemical mechanisms underlying the selective catalytic reduction process are as follows: EQU 4NH.sub.3 +4NO+O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O EQU 4NH.sub.3 +2NO.sub.2 30 O.sub.2 .fwdarw.3N.sub.2 +6H.sub.2 O
As the title of the process suggests, a suitable catalyst is employed to facilitate the desired chemical reduction process. As always, of course, the catalyst itself does not become involved in the reaction but rather it merely accelerates the reduction of the NO.sub.x into nitrogen and water.
Various catalysts of both noble and non-noble metal compositions for use in the selective reduction of NO.sub.x have been suggested. For example, platinum and copper oxide (to name just two) have been mentioned.
Furthermore, there are several types of reactors for carrying out the process. One type of catalytic reactor utilizes catalyst beds which may be fixed, moving or fluidized. Another design employs well-defined flow passages whose boundaries are porous, non-catalytic surfaces which envelop the catalyst and through which the reactants and products diffuse. Alternatively, the surfaces themselves may be comprised of the catalyst.
A review of the relevant art indicates that prior art selective catalytic reduction processes are independent of and separate from the particulate collecting apparatus positioned downstream gas flow-wise from a combustion chamber. However, as a result of the combustion process, many power plants also emit large quantities of particulate matter (ash, soot, dust, unburned carbon, etc.) that must be trapped and collected before the flue gas is permitted to exit to the atmosphere. Various individual components have been successfully utilized in the past, i.e. cyclone collectors, wet scrubbers, electrostatic precipitators and bag houses.
The employment of several air pollution abatement devices in seriatim is expensive from both space utilization considerations and from labor considerations.
Clearly, a single device capable of treating a myriad of pollution problems is desirable.